This invention generally relates to pumps, and in particular, to an apparatus and method for using a jet pump to cool a variable displacement pump.
Variable displacement, axial piston pumps are widely used in aircraft hydraulic systems. During certain flight conditions, the pump will remain in a neutral pumping mode for long periods of time. In neutral, the pump maintains a predetermined system pressure, but pumps only enough fluid to make up system leakage. Hence, the flow of fluid through the pump during its neutral pumping mode is relatively low. In some applications, the normal, high pressure leakage within the pump is insufficient to cool the pump and the hydraulic system.
One solution for cooling the pump and system has been to introduce a predetermined amount of leakage from the pump discharge to the pump casing. One disadvantage of that solution is the additional leakage reduces the overall efficiency of the pump. Another disadvantage is that the energy released by the additional leakage is transferred into heat as the pressure of the fluid drops from the relatively high discharge pressure to the lower casing pressure.
A desirable solution would be to introduce the relatively lower pressure inlet oil into the pump case in order to cool it. However, the pump case fluid is normally at a pressure greater than the inlet fluid so that the inlet fluid will not flow into the case without assistance.
Others have recognized the desirability of using inlet fluid to cool a pump and have provided auxiliary mechanical pumping means in order to achieve that result. See, for example, U.S. Pat. Nos. 4,013,384 and 2,933,044. In the former patent, there is described a centrifugal pumping device which includes cooling passages that are supplied with inlet fluid that is drawn into the pump by the pump's impeller. The latter patent describes a water pumping device which includes an auxiliary impeller to force inlet water through the pump in order to cool it. Still others have used jet pumps for surcharging a pump inlet. See, for example, U.S. Pat. Nos. 4,033,706; 3,989,628; and 3,773,437.